It remains generally accepted that colorectal cancer (CRC) results from the accumulation of genetic events within the epithelial compartment. Unresolved issues include (1) whether the order of these genetic events matter (e.g., APC vs KRAS), (2) whether cancers can initiate from multiple distinct cells of origin, (3) the relationship between the normal colonic stem cell and the cells that function to maintain the tumor and (4) the contribution of the local environment to the neoplastic process. We hypothesize that colonic tumor initiation and progression is highly dependent upon the origin of the cell that incurs the relevant mutagenic events, as well as the regulation of growth and clonal selection of particular progeny that emanate from tumor-initiating cells. We have demonstrated that there are distinct normal colonic progenitor populations marked by expression of two specific cell surface markers, Lrig1 and Lgr5. Although the precise relationship between cells expressing these two markers is unknown, these two bona fide stem cell markers exhibit overlapping, yet distinct, expression patterns in both normal colon and in ApcMin/+ tumors. Of note, Lrig1 is induced by EGFR signaling and acts to negatively regulate EGFR signaling, whereas Lgr5 is a canonical Wnt target of unknown function. We have generated inducible Lrig1-CreERT2 mice, and our collaborator Hans Clevers has provided us with inducible Lgr5-EGFP-IRES-CreERT2 animals that allow for the specific activation of Cre recombinase within distinct colonic stem cell populations. We will use these mice to determine the importance of cell-of-origin in colon cancer initiation and progression. In addition, we have identified a gene, Slc26a3, that is selectively expressed in the differentiated compartment of the colon, and we have generated Slc26a3-CreERT2 mice that will allow us to resolve whether colonic tumors arise from the top down or bottom up.